Be visible, day or night with the super-powerful Specialized Flux Expert Taillight. It delivers 110 lumens for visibility over a 1/3 of a mile in broad daylight. Four modes: Standby, Surge Glow, Daytime Flash, and Nighttime flash let you adapt the Flux for your ride. It features a long lasting lithium ion battery and flip-out USB charger for added convenience. And, the Flux Expert features an ambient light sensor that drops flash level by 50% in total darkness for courtesy to other traffic. This excellent light features a quick-release, dovetail mounting system that fits on almost any seatpost.